Classification of the Spongida. 183 



project, the former arranged vertically, the latter massively. 

 Forms massive, globular, conical, hemispherical, sessile ; or 

 sessile, spreading, simple or appendiculate, branched, stipitate. 



Family 3. Pachytragida. 

 Group 14. G-eodina. 



Massive, incrusting ; or globular and lobed, sessile ; or glo- 

 bular and tree or floating. Structure hard externally, more or 

 less reticulate, radiated, and soft internally ; may be divided 

 into a cortical, a zonular, and a body- or internal substance. 

 Cortex consisting of a hard, stony crust composed of close- 

 packed little siliceous bodies of a globular or ellipsoidal shape, 

 sometimes more or less compressed, even to discoid thinness ; 

 radiated in structure, becoming when mature solid and crystal- 

 line throughout, presenting an ornamented surface, in one part 

 of which is a hiliform depression ; combined with minute stel- 

 lates or minute, sinuous, spined, bacilliform spicules; or straight 

 ones more or less inflated in the centre, skittle-like, and spined; 

 or minute, curved acerates more or less inflated in the centre 

 and microspined ; or minute, simple acerates or acuates, — all of 

 which are flesh-spicules. Zone consisting of a subjacent cor- 

 tical mass of spicules arranged parallel to each other and ver- 

 tical to the body or internal substance, respectively support- 

 ing and piercing the cortex, composed of : — l,the "zone-spicule" 

 par excellence, very robust, consisting of a shaft and three 

 arms expanded under the cortex, which it thus tends to sup- 

 port ; 2, the body- or staple spicule of the whole surface, con- 

 sisting of a large, smooth, more or less curved acerate, which 

 pierces and passes through the cortex for a short distance ; 3, 

 the anchoring-spicule, consisting of a very long delicate shaft 

 and three short terminal arms, which are extended fork-like or 

 recurved anchor-like, more or less plentifully mixed with the 

 foregoing, and often traversing the cortex for some distance, 

 for anchoring ; heads often broken off outside, and there- 

 fore only found perfect, for the most part, inside the cor- 

 tex ; when outside, the fork-like form has often four prongs, 

 opposite, expanded. Body-substance consisting of an areolar 

 sarcodic structure more or less charged with the large acerate 

 spicule before mentioned, together with more or less of the 

 flesh-spicules of the species. Surface even, granular, semi- 

 transparent, and grey when fresh, opaque white when dry. 

 Vents well marked, scattered singly or in groups. Pores in 

 the depressions or small pits of the cortex. Excretory canal- 

 system largely developed, giving the body-substance an open 

 cancellous structure. Texture compact, stony on the surface, 



